1. Field of the Invention
The invention generally relates to transmitter and/or power amplifiers, more specifically to transmitters and power amplifiers operating in at least high band and low band frequencies, and even more specifically to methods and circuits for preventing, or otherwise reducing, the adverse effects of harmonic low band frequencies reaching the output of the transmitter or the power amplifier.
2. Prior Art
In transmitter modules and/or power amplifier (PA) modules there are two problems that may impact their performance. One is the frequency cross band where the second or even the third harmonics of the low band crossover the high band and reach the antenna port. The extent to which this is prevented is referred to as the cross band isolation of the circuit. The better the cross isolation the better the performance of the transmitter or PA module. The other problem is that of the input signal reaching the antenna port by crossing over the transmitter or PA module, and hence forward isolation is required.
The path of each of the forward crossover (170) and band crossover (160) are shown in FIG. 1. A transmitter circuit 100 is comprised of a high band (HB) PA 110 and a low band (LB) PA. The HB PA 110 is coupled to a HB filter 120 and the LB PA 130 is coupled to a LB filter 140. The HB filter 120 and the LB filter 140 are coupled to a switch 150 that routes the signal from one of the HB path or the LB path to an antenna. The LB crossover path (160) is a result of higher harmonic frequencies of the LB frequency fLB, for example 2*fLB and 3*fLB that are passed through the HB PA 110 and HB filter 120 to the switch 150 and therefrom to the antenna, and being an unwanted result. The forward crossover path 170 demonstrates the case of an input frequency crossing over the LB PA 130 and the LB filter 140 finding their way to the antenna path, again being an unwanted result.
While prior art suggests various ways to reduce such adverse effects there is always room for improvement in this area. Therefore, in view of existing deficiencies in the prior art, it would be advantageous to provide an improved solution for both the forward isolation and the cross band isolation in transmitter and PA modules.